1. Field of the Invention
The present invention relates to a speed change control device for a continuously variable transmission for performing a continuous speed change by moving track rings.
2. Description of the Prior Art
Heretofore, there has been well known a continuously variable transmission having a carrier adapted to rotate about an axis thereof, a twin cone adapted to rotate integrally with the carrier while being held by the carrier and rotatable about a central axis intersecting the said axis, and a pair of track rings which are in frictional engagement with the outer peripheral surface of the twin cone, in which the track rings are moved along the axis of the carrier to change their position of engagement with the outer peripheral surface of the twin cone thereby performing a continuous speed change. In such a continuously variable transmission, a feed screw having left-hand threads on one side and right-hand threads on the other side is disposed in parallel with the axis of the carrier, and one track ring is engaged with the left-hand thread portion of the feed screw, while the other track ring is engaged with the right-hand thread portion of the feed screw. And by rotating the feed screw about its axis, the paired track rings are moved in directions approaching each other or in directions going away from each other at the same speed by the same distance.
Generally such feed screw is rotated manually or by means of a motor through gears. But, it is necessary to provide a large space for mounting a gear mechanism which transmits the rotation of the motor or other drive means to the feed screw. This has been an obstacle to the reduction in size of the continuously variable transmission.
As means for moving track rings, utilization of oil pressure without using the above feed screw has also been proposed (see Japanese Patent Publication No. 500569/82 . . . International Application PCT US80 00449 and Japanese Patent Publication No. 500981/82 . . . International Application No. PCT US80 03369). In the use of oil pressure, however, it is difficult to move a pair of track rings equally, and a difference in the amount of movement between both track rings would cause slip at the portion of frictional engagement of the track rings with the outer peripheral surface of the twin cone, which makes it impossible to perform an accurate speed change control and which may cause damage of the device.